Anderson's peat bog zonation

Anderson established his stratigraphic zonation of raised peat bogs based on C₁₄ age determinations, the degree of humification, pollen descriptions and lithology (clay, wood peat, fibrous peat and remains of leaf mosses).

A section logged by him to the east of Marudi illustrates the stratigraphic succession:

The base of the section, at a depth of 13-11.5 metres, consists of a very humic clay with semi-decayed plant fragments, possibly derived from Nypa palms. The pollen found in this clay indicate a mangrove sediment. Direct above this clay is a transition zone into peat. From 11.5 to 5.5 metres the peat is rather compact and consists mainly of strongly humified wood with a crumbling granular texture. From 5 to 2 metres the peat is remarkably different, with few wood remains and a very high water content; sloppy and less humified. The upper two metres consist of heavily compacted, fibrous and highly humified peat with few small pieces of wood. The main constituents are roots, rootlets and cuticles. The section represents a gradual change from brackish to marine interfluvial deposition at the base through freshwater peat swamps to raised mire peat deposition at the top. Initial peat accumulation starts in brackish to marine interfluve mudflat areas, which are quickly colonised by mangrove and Nypa vegetation. Peat in this setting is decomposed by microbial activity, high in ash and high in sulphur (iron sulphides). In the prograding delta system, subsequent extensive mangrove and Nypa belts filter sediments and limit flooding by brackish or marine waters (e.g. the mangrove swamp along the southeastern side of Brunei Bay), thus creating and protecting freshwater peat development further inland. In the upper delta plain environment, where fluvial channels accrete laterally within the confines of the flood plain and intermittently cut into swamps, the peat deposits are laterally discontinuous. Further downstream, in the lower delta plain, peat deposits tend to be more continuous.
The brackish to marine interfluve peat is initially moderately decomposed and high in ash content. Increased water acidity and reduced communication with more saline waters results in lower sulphur content. The freshwater peat swamps subsequently develop into raised mires with an elevated ground water table as a result of high organic accumulation rates. The resulting peat deposits are laterally extensive, low in ash and sulphur. Staub and Esterle (1994) discuss raised mire, peat-accumulation systems in the modern Baram, Rajang and Lupar deltas along the west coast of Sarawak and Brunei Darussalam. They argue that sediment supply and tectonic setting control variations in these three fluviomarine systems. These factors determine primarily the depositional system geomorphology and affect the geometry of the domed peat deposits. They do not influence their internal compositional characteristics from basal high ash, high sulphur degraded peats overlain by low ash, low sulphur peats. Raised mires, because of their solid consistency, are remarkably resistant to erosion.

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